Each cell within a cellular mobile communication system is serviced by a base station. When it is switched on, a mobile station selects the base station with which it will secure the best radio link. If the mobile station is moved and changes cell, it can than re-select another base station. This selection/reselection process takes place with the aid of a beacon signal which is transmitted on a specific frequency by each base station.
In many systems, the changeover of cell may also take place during a call, in which case it will be managed by a process known as <<handover>>. The base station providing the radio link to the mobile station sends data to it, identifying the channels on which the base stations of neighbouring cells are transmitting their beacon signals. This information may consist of a list of frequencies relating respectively to the beacon signals of adjacent cells. In parallel with the communication, the mobile station monitors these frequencies. A comparison is run between the reception conditions in the cell currently being serviced and in the adjacent cells so that a decision can be taken as to the time at which the changeover of cell should be effected. Depending on the systems, this decision is taken either at the level of the mobile stations or at the level of the fixed infrastructure.
In confined environments, in which radio waves are not readily propagated, the continuity of cellular mobile communication services is sometimes provided by means of transmission cables installed along the zones where service has to be provided. These cables are generally of the coaxial type, having an imperfect external shield so that losses in radiation will provide radio coverage in the zone in question. These are used in railway tunnels in particular. Taking the example of an underground railway system, the base stations of the cellular system are positioned in stations, typically spaced apart by 500 to 1500 meters, and linked to loss cables extending along the tunnels between stations.
Using loss cables poses a problem when it comes to the handover procedure because the field emitted by such a cable fades abruptly at the ends of the cable. The successive runs of cable are usually placed end to end which means that there is only very low coverage of the cells at the boundaries. Under these conditions, the mobile station does not have time to perform the steps needed with respect to the beacon signal of the adjacent cell to be able to run the handover, particularly as the mobile station is generally moving at quite a high speed (in the example of the underground railway, the boundary between two cells is usually in the middle of a tunnel between two stations, i.e. at a point where the rolling stock is travelling at full speed). Consequently, there is a risk that the call will be cut off due to the fact that the mobile station has not been able to run the requisite steps before entering a new cell.
In order to remedy this problem, publication WO97/16 892 proposes a system of overlapping successive lengths of cable belonging to two neighbouring cells over a certain length at the boundary of these cells, one of the two lengths being provided with an attenuator positioned so that it will attenuate the radiation of one of the two cables in the overlap zone. This attenuation is such that the mobile station is able to continue communicating with the base station feeding the cable equipped with the attenuator when directed towards the other cell, whilst running the necessary steps to operate the handover on the beacon signal transmitted from the other cell without any attenuation. The disadvantage of this method is that the level of the signals, already low at the ends of the cable prior to attenuation, become unusable for practical purposes once attenuated unless the overlap between the runs is relatively long. However, the cost of installing a long overlap is high, given the price of loss cables. Furthermore, the method is uni-directional: in the case of a tunnel in which trains are travelling in the two directions, handovers are only possible in one direction. Furthermore, installation requires work to be carried out in the middle of the tunnels if the railway network is equipped with preexisting loss cables, which complicates the installation process.
Another drawback of this solution is the fact that the attenuation is applied to all the signals carried by the cables. Very often, these cables carry several mobile communication services, provided by different cellular systems and/or run by different operators. The method is therefore not suitable if only one operator wants to use it.